The present invention relates to a method and a device for automatic protection of an aircraft against a risk of collision with the ground, this method and this device using a protection function that prevents an aircraft from colliding with the ground.
Patent FR 2 986 876 describes an automatic method for protecting an aircraft, in particular a transport airplane, against a risk of collision with the overflown terrain. This method implements automatic protection, called GCoP (for ‘Ground Collision Protection’), which uses a prohibited flight envelope and which prevents the aircraft from touching the ground under certain conditions. More precisely, this automatic protection prevents the aircraft from flying under a given height with respect to the ground (generally 200 or 250 feet).
As part of this GCoP protection, a non-operational and critical situation is considered that could lead to a CFIT collision (for ‘Controlled Flight Into Terrain,’ a term referring to a collision with the ground not following from a failure, or from a loss of control of the aircraft), and a prohibited flight envelope is used that avoids a collision with the ground. The prohibited flight envelope (defined by vertical speed and altitude pairs) can be determined from technical characteristics of the aircraft and the structure thereof.
When the aircraft is in a prohibited flight envelope, protection orders (overriding the authority of the pilot) are generated. These protection orders are aimed, in particular, at automatically controlling the control surfaces of the aircraft which act on the vertical speed of the aircraft. These protection orders are such that, when they are applied to these control surfaces, the aircraft emerges from the prohibited flight envelope.
Protection orders, which are applied as long as the GCoP protection function is activated (or enabled) and it is engaged (i.e., the aircraft has a vertical speed/altitude pair in the prohibited flight envelope), are used to progressively reduce the rate of descent of the aircraft until, for example, bringing it down to a zero vertical speed, thus preventing the aircraft from descending and therefore colliding with the overflown terrain.
When the conditions of engagement are no longer met, the GCoP protection function is disengaged and the crew regains its nominal authority, i.e., its commands are again taken into account and no longer the commands of the protection orders.
In the context of the present invention, it is considered that:                the protection function is activated (or enabled) if it is brought into a situation in which it is ready to automatically implement the protection provided, if particular conditions (of engagement) are met (in particular the aircraft entering a prohibited flight envelope). If it is not in this situation, it is considered to be deactivated so that the protection is not implemented even if the conditions of engagement are met; and        the protection function is engaged, if it implements the protection provided, i.e., if it determines and applies the protection orders. Consequently, for being able to be engaged, the protection function must first be activated, then it is engaged (and provides protection) if particular conditions are met.        
The GCoP protection function can be activated during a landing under certain conditions.
When the GCoP protection function has been activated, it is necessary to deactivate it in order to be able to perform the landing if the aircraft is in a situation allowing a landing, in particular if it regains a correct stabilized flight path toward the landing strip, for avoiding its engagement.